It is known that polyisocyanates can be reacted in the presence of phosphorus-containing catalysts to form polyisocyanate-carbodiimides which are generally substantially insoluble in organic solvents (see, e.g., U.S. Pat. No. 2,941,966). This reaction is accompanied by the evolution of carbon dioxide. The polyisocyanate-carbodiimides thus obtained are partly branched or even crosslinked by adduct formation of carbodiimide groups with isocyanate groups to form uretidindione imines, or even by dimerization of carbodiimide groups to form uretidindione diimines. It is also known that carboxylic acids can be reacted in an addition reaction with isocyanates to form amides and with carbodiimides to form acyl ureas.
Prepolymeric polyisocyanates containing acylated urea groups obtained by the addition of carboxyl groups to carbodiimide groups have not as yet been reported. In fact, it would not be expected that compounds of this kind could be obtained by the addition of carboxyl groups to polyisocyanates containing carbodiimide groups because the undesirable addition reaction between carboxyl and isocyanate groups (i.e., undesirable for the production of these compounds) would be expected to accompany the desirable addition reaction between carbodiimide groups and carboxyl groups. In fact, carboxylic acid anhydrides are known to be formed by the reaction of compounds containing carbodiimide groups with carboxylic acids (see, e.g., F. Kurzer and K. Douraghi-Zadeh, Chem. Reviews 64 107 (1967)).